Bottom Line:
Furthermore, we observed that the late stage of cluster growth is accompanied by the formation of circular depletion zones around the largest clusters.From the quantification of the evolution of the size of these zones, the Au surface diffusion coefficient was evaluated in D(T) = [(7.42 × 10-13) ± (5.94 × 10-14) m2/s]exp(-(0.33±0.04) eVkT).As a consequence we acquired a methodology to control the morphological characteristics of the Au film simply controlling the annealing temperature and time.

ABSTRACTThe study of surface morphology of Au deposited on mica is crucial for the fabrication of flat Au films for applications in biological, electronic, and optical devices. The understanding of the growth mechanisms of Au on mica allows to tune the process parameters to obtain ultra-flat film as suitable platform for anchoring self-assembling monolayers, molecules, nanotubes, and nanoparticles. Furthermore, atomically flat Au substrates are ideal for imaging adsorbate layers using scanning probe microscopy techniques. The control of these mechanisms is a prerequisite for control of the film nano- and micro-structure to obtain materials with desired morphological properties. We report on an atomic force microscopy (AFM) study of the morphology evolution of Au film deposited on mica by room-temperature sputtering as a function of subsequent annealing processes. Starting from an Au continuous film on the mica substrate, the AFM technique allowed us to observe nucleation and growth of Au clusters when annealing process is performed in the 573-773 K temperature range and 900-3600 s time range. The evolution of the clusters size was quantified allowing us to evaluate the growth exponent 〈z〉 = 1.88 ± 0.06. Furthermore, we observed that the late stage of cluster growth is accompanied by the formation of circular depletion zones around the largest clusters. From the quantification of the evolution of the size of these zones, the Au surface diffusion coefficient was evaluated in D(T) = [(7.42 × 10-13) ± (5.94 × 10-14) m2/s]exp(-(0.33±0.04) eVkT). These quantitative data and their correlation with existing theoretical models elucidate the kinetic growth mechanisms of the sputtered Au on mica. As a consequence we acquired a methodology to control the morphological characteristics of the Au film simply controlling the annealing temperature and time.

Mentions:
To understand the formation of the Au clusters, first of all, we analyzed the morphology of the starting Au film after the 573 K-900 s. So, Figure 3a,b shows 20 μm × 20 μm and 10 μm × 10 μm AFM images of the Au film annealed at 573 K-900 s. Interestingly, we observe that this annealing process determines the nucleation of small Au clusters (height of about 10 nm) from the starting quasi-continuous film. Furthermore, while the nucleation of these small clusters takes place, also the formation of small holes (depth of about 10 nm) in the Au film occurs. Figure 4 reports, also, 1 μm × 1 μm AFM images of the same sample focusing both on the small Au clusters and the holes. Figure 4b shows an AFM cross-sectional line scanning profile analysis that refers to a Au cluster imaged in Figure 4a: the section analyses allow to evaluate its height in 11.2 nm. Similarly, Figure 4d shows the AFM cross-sectional line scanning profile analysis that refers to an hole imaged in Figure 4c, allowing to evaluate its depth in 7.4 nm. We can conclude that the 573 K-900 s annealing process determines the first stage of nucleation of Au clusters from the starting quasi-continuous film and that the following annealing processes cause their growth. To study the growth stage, we imaged by the AFM the Au clusters annealed between 573 and 773 K and 0-3600 s at higher resolution. As examples, Figure 5 reports 50 μm × 50 μm AFM images of the starting Au film subjected to various thermal treatments: (a) 573 K-1800 s, (b) 673 K-3600 s, and (c) 773 K-3600 s. The qualitative increase of the mean clusters size and the decrease of their surface density increasing the annealing time t and/or temperature T are evident. The main feature in the late stage of the cluster growth is the formation of circular depletion zones around the largest clusters. We used the AFM analyses, also, to image the morphology structure of the large clusters and of the depletion zones around them. So, for examples, Figure 6a shows a 7 μm × 7 μm AFM image of a single Au large cluster (corresponding to the 673 K-3600 s annealed sample), while Figure 6b shows a 1 μm × 1 μm AFM image of depletion zone near the cluster, and Figure 6c shows a 1 μm × 1 μm AFM image taken over the Au cluster. Figure 6b shows a percolation morphology of the underlaying residual Au film (similar to that of the starting 28 nm Au film), while Figure 6c shows a more complex nano-structure: the large cluster appears to be formed by Au nanoclusters.

Mentions:
To understand the formation of the Au clusters, first of all, we analyzed the morphology of the starting Au film after the 573 K-900 s. So, Figure 3a,b shows 20 μm × 20 μm and 10 μm × 10 μm AFM images of the Au film annealed at 573 K-900 s. Interestingly, we observe that this annealing process determines the nucleation of small Au clusters (height of about 10 nm) from the starting quasi-continuous film. Furthermore, while the nucleation of these small clusters takes place, also the formation of small holes (depth of about 10 nm) in the Au film occurs. Figure 4 reports, also, 1 μm × 1 μm AFM images of the same sample focusing both on the small Au clusters and the holes. Figure 4b shows an AFM cross-sectional line scanning profile analysis that refers to a Au cluster imaged in Figure 4a: the section analyses allow to evaluate its height in 11.2 nm. Similarly, Figure 4d shows the AFM cross-sectional line scanning profile analysis that refers to an hole imaged in Figure 4c, allowing to evaluate its depth in 7.4 nm. We can conclude that the 573 K-900 s annealing process determines the first stage of nucleation of Au clusters from the starting quasi-continuous film and that the following annealing processes cause their growth. To study the growth stage, we imaged by the AFM the Au clusters annealed between 573 and 773 K and 0-3600 s at higher resolution. As examples, Figure 5 reports 50 μm × 50 μm AFM images of the starting Au film subjected to various thermal treatments: (a) 573 K-1800 s, (b) 673 K-3600 s, and (c) 773 K-3600 s. The qualitative increase of the mean clusters size and the decrease of their surface density increasing the annealing time t and/or temperature T are evident. The main feature in the late stage of the cluster growth is the formation of circular depletion zones around the largest clusters. We used the AFM analyses, also, to image the morphology structure of the large clusters and of the depletion zones around them. So, for examples, Figure 6a shows a 7 μm × 7 μm AFM image of a single Au large cluster (corresponding to the 673 K-3600 s annealed sample), while Figure 6b shows a 1 μm × 1 μm AFM image of depletion zone near the cluster, and Figure 6c shows a 1 μm × 1 μm AFM image taken over the Au cluster. Figure 6b shows a percolation morphology of the underlaying residual Au film (similar to that of the starting 28 nm Au film), while Figure 6c shows a more complex nano-structure: the large cluster appears to be formed by Au nanoclusters.

Bottom Line:
Furthermore, we observed that the late stage of cluster growth is accompanied by the formation of circular depletion zones around the largest clusters.From the quantification of the evolution of the size of these zones, the Au surface diffusion coefficient was evaluated in D(T) = [(7.42 × 10-13) ± (5.94 × 10-14) m2/s]exp(-(0.33±0.04) eVkT).As a consequence we acquired a methodology to control the morphological characteristics of the Au film simply controlling the annealing temperature and time.

ABSTRACTThe study of surface morphology of Au deposited on mica is crucial for the fabrication of flat Au films for applications in biological, electronic, and optical devices. The understanding of the growth mechanisms of Au on mica allows to tune the process parameters to obtain ultra-flat film as suitable platform for anchoring self-assembling monolayers, molecules, nanotubes, and nanoparticles. Furthermore, atomically flat Au substrates are ideal for imaging adsorbate layers using scanning probe microscopy techniques. The control of these mechanisms is a prerequisite for control of the film nano- and micro-structure to obtain materials with desired morphological properties. We report on an atomic force microscopy (AFM) study of the morphology evolution of Au film deposited on mica by room-temperature sputtering as a function of subsequent annealing processes. Starting from an Au continuous film on the mica substrate, the AFM technique allowed us to observe nucleation and growth of Au clusters when annealing process is performed in the 573-773 K temperature range and 900-3600 s time range. The evolution of the clusters size was quantified allowing us to evaluate the growth exponent 〈z〉 = 1.88 ± 0.06. Furthermore, we observed that the late stage of cluster growth is accompanied by the formation of circular depletion zones around the largest clusters. From the quantification of the evolution of the size of these zones, the Au surface diffusion coefficient was evaluated in D(T) = [(7.42 × 10-13) ± (5.94 × 10-14) m2/s]exp(-(0.33±0.04) eVkT). These quantitative data and their correlation with existing theoretical models elucidate the kinetic growth mechanisms of the sputtered Au on mica. As a consequence we acquired a methodology to control the morphological characteristics of the Au film simply controlling the annealing temperature and time.